1. Field of the Invention
The present invention relates to an improvement of a heat transfer surface for heat transfer between a heat source and a coolant.
2. Description of the Prior Arts
The heat transfer using a boiling liquid coolant will be illustrated.
As it is well-known, the heat quantity Q (Kcal/h) transferred from heat transfer surface to the liquid contacted with the heat transfer surface can be given by the equation: EQU Q=.alpha..multidot.A.multidot..DELTA.T (1)
wherein .alpha. designates a heat transfer coefficient (Kcal/m..sup.2 h..degree.C.) given by boiling; A designates a surface area (m.sup.2) of the heat transfer surface; and .DELTA.T designates a temperature difference between a surface temperature Tw(.degree.C.) of the heat transfer surface and a temperature T(.degree.C.) of the liquid.
The heat transfer surface having good heat transfer characteristics means the heat transfer surface which transfers large heat quantity Q from the heat transfer surface to the liquid in a small temperature difference .DELTA.T. Thus, the heat transfer surface having large value of .alpha. x A is the heat transfer surface having good heat transfer characteristics in view of the equation (1).
Heretofore, in order to increase the heat transfer surface area A, fins have been formed on the heat transfer substrate or a rough surface has been formed by a sandblast.
In order to increase the heat transfer coefficient .alpha., a porous surface has been used from the following viewpoint.
The heat transfer in the boiling phenomenon is controlled by the behavior of the liquid in the local region near the heat transfer surface. The boiling heat transfer coefficient .alpha. is remarkably greater than that of the convection heat transfer having no phase change without forming steam, because of the stirring effect caused by bubbling of steam generated and leaving from the heat transfer surface and the latent heat transfer effect. For example, the forced convection heat transfer coefficient of air can be only several tens to several hundreds (Kcal/m..sup.2 h..degree.C.) whereas the boiling heat transfer coefficient of water can be several thousands to several ten thousands (Kcal/m..sup.2 h..degree.C.). The steam bubbles are formed by boiling the liquid contacted with the heat transfer surface. When the steam bubbles are generated and left from the heat transfer surface, fresh liquid should be fed on the heat transfer surface. Otherwise, the heat transfer surface is dried to be covered with the steam whereby a film boiling condition is caused and the heat transfer coefficient .alpha. is suddenly decreased. Thus, in order to increase the boiling heat transfer coefficient .alpha., the number for bubble forming points on the heat transfer surface should be increased and the smooth feeding of the liquid on the heat transfer surface should be given. On a porous surface, the steam in many cavities results bubble nuclei and the cavities are connected in the porous layer, fresh liquid is fed to the bubble nuclei. Thus, the heat transfer coefficient .alpha. can be increased.
FIG. 1 shows the heat transfer surface considered by the conventional consideration. A porous layer (3) is formed on the surface of the smooth heat transfer substrate by sintered metal (1). The porous heat transfer surface (4) is formed by the porous layer (3). On the porous heat transfer surface (4), many cavities (5) are formed in the porous layer (3) and the steam is kept in the cavities (5). It is necessary to form bubble nuclei in order to generate steam bubbles from the heat transfer surface and to leave into the liquid (6). In the porous surface, the steam in the cavities (5) can be bubble nuclei. The bubble nuclei are grown by the heating of the heat transfer surface so as to form steam bubbles.
On the smooth heat transfer surface, the bubble nuclei may be in scratches or cracks on the smooth heat transfer surface. The number of scratches or cracks is remarkably smaller than the number of bubble nuclei in the porous heat transfer surface (4). Thus, the formation of the steam bubbles is small whereby the heat transfer coefficient .alpha. is remarkably smaller than that of the porous heat transfer surface (4).
On the porous heat transfer surface (4), cavities (5) are connected in the porous layer (3). When local active bubbling nuclei are formed, the fresh liquid is continuously fed from the other poor bubbling centers to the local active bubbling nuclei. The feeding of the fresh liquid is promoted by capillary effect in the porous layer (3).
The porous heat transfer surface (4) has the above-mentioned advantages to be suitable as a boiling heat transfer surface. However, the preparation of the conventional porous heat transfer surface (4) using the sintered metal (1) is not easy. That is, in the preparation, metal particles are mixed with a binder such as phenol resin and the mixture is coated on the surface of the heat transfer substrate (2) and they are heated at high temperature to sinter the metal particles on the surface of the heat transfer substrate (2) and it is further heated to remove the binder by a reduction after the sintering.
Thus, in the preparation of the conventional porous heat transfer surface, the control of the atmosphere in the sintering or the control of the binder is not easy. Moreover, the metal particles are melt-bonded and accordingly, the structure of the porous layer is complicated. The complicated process control is disadvantageously required in a mass production of uniform products.